Protein crystal growth under microgravity conditions results in substantially increased crystal size and quality. The application of a microgravity environment is the subject of several ongoing investigations which aim to increase the size and internal order of protein crystals. Numerous successful applications of a microgravity environment to the growth of high quality protein crystals have been well documented. This is extremely important, since the ability to produce high quality protein crystals has been the limiting step in a number of important macromolecule structural problems. Crystallization of proteins is an important requirement in determining protein structure. Protein crystallography is used to ascertain the three-dimensional molecular structure of protein crystals. This is essential for understanding the biological functions attributed to these macromolecules. The physical shape and folding of a protein is of increasing importance to drug companies interested in rational drug design. Drug molecules are designed to fit exactly into a binding site of a macromolecule, thus blocking its function in a given disease pathway. Producing higher quality crystals results in more accurate modelling of the 3-dimensional protein structures, and consequently more efficacious drugs can be produced. This accuracy is referred to as the resolution of the structure. The larger and more perfect crystals provide the highest resolution.
Presently, the field of macromolecular crystallography is undergoing a major technological revolution which permits more efficient and difficult structure determinations. These improvements coupled with the advances in recombinant technologies are providing an increase in the number of structures determined yearly. Typically, the growth of large, high quality protein crystals using ground-based methods requires numerous crystallization surveys to identify and maximize the proper growth conditions.
With regard to making a crystal (preferably a single crystal structure), various methods have been conventionally known. For example, a melt growth method in which a melt is slowly solidified and a solution growth method in which a solution of raw materials is cooled gradually have been provided. To avoid crystalline defects or in-homogeneity in composition and make a perfect single crystal, a zero-gravity environment such as in space or orbit has been utilized. For instance, a melt or solution is cooled under a floating state without using any container, and a crystal is made. This method is called a container-less method, and it is a stable method for making a highly pure crystal because any contamination from a container can be completely avoided. A large crystal is also made because a melt of large size can be supported without any container under a zero-gravity environment. At the beginning of crystal growth experiment in space, a perfect crystal without any defect or in-homogeneity was expected to be realized on the grounds that convection due to gravity does not occur and crystal growth proceeds in a melt or solution without any influence of disturbance.
One important field of such a space experiment is the growth of protein crystals under a microgravity environment. The growth of protein crystals is an important as well as fundamental step for determining the molecular structure and for investigating the relationship between the structure and function of protein molecules. Based upon the determination of the molecular structure, proteins can be designed to have a desired function. This is one of the major goals of protein engineering.
In view of the foregoing problems, experiments in space for growing protein crystals under microgravity conditions attracts the attention of various researchers, as such a microgravity environment does not cause convection during growth of the protein crystals. Perfect crystals are difficult to achieve on Earth. Ambient gravity and turbulence disrupt crystal formation, in that terrestrial samples mix as a result of gravity-driven convective flow. Therefore a microgravity environment promotes better crystal formation, in part due to the lack of turbulence and mixing within a liquid or gaseous sample during crystal formation. Spacecraft in low Earth orbits can provide a microgravity environment that is convection- and sedimentation-free for the study and application of fluid-based systems. With the advent of the Space Shuttle, scientists have regular access to such environments and many experiments have been initiated, including those in protein crystallization. After many trials it became clear that, for several proteins, crystallization in a microgravity environment resulted in bigger and better quality crystals. The generation of perfect crystals can sometimes be the limiting factor in determining a protein's structure. By eliminating variables such as gravity, crystals are able to form more slowly and more precisely in space. Temperature can be a significant variable in biological macromolecule and small molecule crystallization. Temperature often influences nucleation and crystal growth by manipulating the solubility and supersaturation of the sample. Thus, the control of temperature during crystal production is essential for successful and reproducible crystal growth of proteins with temperature dependent solubility. An advantage is that a temperature gradient provides precise, quick, and reversible control of relative supersaturation. In addition to standard crystallization variables (such as sample concentration, reagent composition and concentration, and pH), temperature variables can increase the probability of producing crystals as well as uncover new crystallization conditions for a sample. Protein solution temperature can be used to carefully manipulate crystal nucleation and growth. This control can also be used to etch or partially dissolve and then grow back the crystal in an attempt to improve crystal size, morphology, and quality. Temperature control is noninvasive and can manipulate sample solubility and crystallization with altering reagent formulation.